The invention relates to an apparatus for guiding and stopping an anchor chain on a floating structure, wherein the chain comprises links extending like a catenary between the structure and an associated anchor, the apparatus comprising a means for guiding standing chain links and guiding and supporting lying chain links, and a stopper for retaining the chain in a desired position.
In anchor systems for the anchoring of floating structures, such as offshore platforms or vessels for hydrocarbon recovery, the anchor chains preferably are terminated above water or easily accessible, whereas they should preferably be guided into the floating structure or vessel under water. Guide wheels are then used to terminate the catenary course, for thereafter to carry the chain further alongside or internally of the structure up to a suitable place for the chain stopper.
In this connection, according to the practice of today, there are used either 1) so-called xe2x80x9cfairleadsxe2x80x9d in the form of blade wheels or guide wheels having pockets which are adapted for receiving individual chain links, or 2) fixed guide disks having a constant radius of curvature as viewed into the catenary plane.
The guide wheels are advantageous in applications wherein frequent anchor line operations occur. The guide wheels have pockets for the lying chain links and grooves for the standing chain links, something which gives a good support for guiding of the chain links. A drawback of the guide wheels is, however, that they become very large.
The fixed guide disks consist of a segment having a fixed radius of curvature. The curvature in the catenary plane may, e.g., have a radius of curvature of about 3 555 mm for a chain having a thickness of 102 mm (see U.S. Pat. No. 4,742,993). The drawback of the fixed guide disk is that the chain links will be xe2x80x9cridingxe2x80x9d on the curved surface of the lying links in the operating condition, something which results in undesirably large stresses in the chain links.
The object of the invention is to provide a guide means for the current purpose which is less space-demanding than the above-mentioned guide wheels, which does not require any bearing as is necessary to rotate a guide wheel, and wherein the construction is designed in such a manner that the additional stresses in the chain links will be small.
For achieving the above-mentioned object there is provided an apparatus of the introductorily stated type which, according to the invention, is characterized in that the guide means comprises a mouthpiece member having an inner guide passage with a generally cruciform cross-section with a first arm having a pair of opposite grooves for the receipt of standing chain links, and a second arm having a pair of opposite grooves for the receipt of lying chain links, wherein each arm has a constant width transversely to the catenary plane of the chain, but diverges in the catenary plane in the direction towards the catenary part of the chain, the opposite grooves of the second arm having lying link supporting surfaces cooperating in pairs and of which at least one pair comprises one or more planar surface portions having a length which is adapted to support individual, lying chain links when the chain is retained in the desired position by means of the chain stopper.
In an advantageous embodiment of the apparatus, each of the opposite diverging supporting surfaces is provided with said planar surface portions. Further, it is advantageous that the planar surface portions have a length corresponding to the nominal chain length plus an additional length determined by the length tolerance of the chain. Further, the planer surface portions of the associated supporting surface preferably are connected to each other through single-curved surface portions.
The guide means according to the invention allows the use of a smaller angular change between adjacent chain links than in a guide wheel (fairlead), so that the equivalent diameter for a corresponding guide wheel becomes larger. A guide wheel usually is executed with seven pockets. With an embodiment in which the guide means according to the invention is mounted in a suitable inclined position adapted to the chain course, one can easily obtain an equivalent diameter corresponding to a guide wheel having twenty pockets.
As will be known to a person skilled in the art, a chain is delivered with a certain length tolerance for the chain links. When considering a guide wheel having teeth which are placed pursuant to the length of the links, one will face one of two cases: In one case the chain is short, and it will then rest against a planar surface in a corresponding manner as in the guide means according to the invention. In the other case the chain is long in relation to the pocket, and the chain then will get jammed between the teeth. This will not happen in the guide means according to the invention. It is therefore likely that the chain links will have better supporting conditions in the present guide means than in a guide wheel according to the prior art.